Most known fluid recovery units are designed to maximize the amount of recovered fluid over a given length of time. These pumps, which pump a relatively large amount of fluid at a relatively high rate, typically operate with a symmetrical upstroke and downstroke. For instance, where such systems are driven by a crank wheel, the upstroke occurs over about 180 degrees of crank wheel rotation as does the downstroke. This scheme is desirable in high volume, high speed recovery systems because the upstroke, i.e., the only portion of unit motion where fluid is actually recovered, should occur at as high a frequency as possible. When the upstroke occurs at a high frequency, more fluid is pumped to the surface. However, the benefit provided by a fast, high frequency upstroke is offset by the costs associated with such an operation. Specifically, such a scheme places substantial shock loads on the recovery unit as the fluid and moving components are required to undergo relatively high accelerations. Moreover, these systems require a large amount of work to be performed in order to lift a higher weight of fluid per unit of time. Nevertheless, these offsetting costs are considered to be acceptable in view of the large amount of fluid that is brought to the surface. In sum, these systems remain profitable as long as an adequate amount of fluid is recovered, despite the large amount of work required to recover the fluid.
However, the considerations mentioned above are not usefully applied to a recovery unit operating at low speed, in a low volume well. In that case, the primary goal is not to recover a maximum amount of fluid per given length of time. Instead, the primary goal is to efficiently recover what fluid is available. Accordingly, a recovery unit operating with a symmetrical upstroke and downstroke is not desirable for a low volume, low speed recovery unit. In such case, the operating costs associated with a high frequency upstroke, namely the shock loads exerted upon the system and the high amount of work performed, outweigh the value of the recovered fluid. Ideally, a recovery unit operating to recover a low volume of fluid would do so in a way that minimizes work performed. As a natural result, such a system would recover fluid so that the recovered fluid and moving components are subject to low accelerations.
Notwithstanding the above, known fluid recovery systems are subject to other additional limitations. Namely, a number of design features found in known recovery systems leave much to be desired. These design features limit system productivity and increase operating costs, particularly where those systems are used to recover fluid while operating at low strokes per minute (SPM).
By way of example, known recovery systems are limited in efficiency because they utilize too many structural bearings. The number of structural bearings in a system is inversely related to mechanical advantage (as each bearing introduces parasitic loads in the form, e.g., friction, heat, etc.). As a natural result of these inefficiencies, known recovery units need intensive maintenance, requiring relubrication every 3 to 6 months. Also, such systems must run above a threshold stroke per minute rate (SPM) to maintain lubrication between working components. As such, these systems cannot be used for low SPM applications.
Known recovery systems are also overly expensive to build and operate as they incorporate a relatively large gearbox, which is required to handle the full torque capacity rating of the recovery unit. Larger gearboxes cost significantly more to manufacture than smaller gearboxes. Some known systems utilize a belt drive between the motor and gearbox, where the gearbox output drives the crank arm. However, such an arrangement requires that the gearbox have a torque capacity at least as large as that of the recovery unit. Other known systems utilize two belt drives but do not achieve additional speed reduction between those drives (e.g., where a flywheel is used between the first and second drive). However, such a combination is particularly undesirable for low SPM operations because of the high rotational inertia effect of the flywheel.
Moreover, the majority of known recovery units typically have symmetrical operating geometries, meaning that it takes about 180 degrees of crankshaft rotation for the recovery unit to move the polished rod up or down in the well. This is often a necessary feature where the unit is operated at a relatively high SPM rate, typically 5 SPM to 15 SPM. However, these high SPM systems waste much energy when a low volume well is being pumped.
Known recovery units tend to suffer from a high cyclic load factor (CLF), which is defined as the quotient of the root-mean-square and average motor current. The CLF can be interpreted as a measure of electrical heating and cyclic loading observed during operation. The symmetrical operating geometry and relatively high speeds associated with known recovery systems (as mentioned above) largely contribute to the high CLF of known systems.
Other systems known in the art operate at a mechanical disadvantage as they utilize either of 1) a dual pitman arm arrangement, or 2) a single pitman arm arrangement that is not counterbalanced. As a result, these systems suffer from side loading on system bearings and/or a propensity to rotate about an axis drawn between the center of the wrist pin and tail bearing. This can cause unusual wear and early failures. Known systems are at a further disadvantage because they require manual adjustments in order to maintain tension across their drive belts. That is, the distance between drive components must be manually adjusted to maintain tension on belts extending therebetween. This is cumbersome and expensive in terms of manual labor and system downtime.